1. Field of the Invention
The present invention relates to a method of production for an active carbon electrode which can be employed as an electrical double layer condenser, and to an active carbon electrode obtained by the method.
2. Description of the Related Art
Electrical double layer condensers are utilized as back-up power sources in electronic devices such as personal computers. The use of electrical double layer condensers as auxiliary power sources for temporarily supplying a large current, as in the case of an auxiliary battery for a car, is also being developed. The electrodes in this electrical double layer condenser are referred to as "polarizable electrodes", and are required to have a large capacitance. A conductive carbon material having a large specific surface area is therefore used as the material for the polarizable electrode. Active carbon is particularly preferable for this purpose. Such active carbon base materials are available in the form of both powders and fibers.
A variety of pyrolytic carbon compounds, from carbon substances such as coke, coal, coconut shell char and the like, as well as thermosetting resins like phenol resin, may be used as the source material in making active carbon.
FIG. 14 is a schematic diagram showing the process for producing active carbon in the case where phenol resin is used as the source material. As shown in this figure, the phenol resin is first cured, and then undergoes dry distillation to volitize non-carbon components, thereby completing the carbonization process. Activation is then carried out, followed by grinding and sieving as necessary to obtain an active carbon base material ranging from a powder to granular form.
Additionally, a method is also known for obtaining a fibrous active carbon base material by using acrylic fibers as the carbon compound source material and carrying out the carbonization process in the same way while maintaining the form of the fibers.
Polarizable electrodes for electrical double layer condensers conventionally have been used in paste form by mixing the active carbon base material, ranging from powder to granular form, with a sulfuric solution. However, a large current could not be conveyed due to the large contact resistance between the active carbon particles. Similarly, when using a fibrous activated carbon cloth, there is low contact resistance between the fibers and the active carbon density per unit volume is small, making it impossible to obtain a large current.
Accordingly, a method such as shown in FIG. 15 may be considered wherein a binder is added to the active carbon base material and molded. The added binder then undergoes carbonization in the same way, followed by sintering to form a molded body in the form of a plate. A pyrolytic carbon compound which, after carbonization, will form a carbon substance identical to that of the base material may be selected for the binder. Accordingly, in the present case, it is preferable to use phenol resin, since it is identical to the source material.
The use of a plate shaped formed body having a large density as a polarizable electrode can be anticipated as well for the case where fibrous active carbon is employed as the base material, provided that the interval of space between the fibers is embedded with the binder, followed by carbonization in the same manner and sintering.
However, in an active carbon polarizable electrode for use as an electrical double layer condenser that is obtained by adding a binder to a conventional active carbon base material as described above and carrying out carbonization and sintering, the capacitance relative to the weight of the electrode was sometimes lower than that of the base material. It is believed that the reason for this is not only because in the conventional method described above, the carbonaceous components formed by the carbonization of the added binder are not activated but also because the activity of the base material is somehow impaired by the carbonaceous components.
Therefore, as shown in FIG. 16, an attempt was made to carry out the activation treatment last, after adding the binder to unactivated carbon powder and forming, rather than carrying out activation prior to obtaining the active carbon base material. However, a base material having sufficient mechanical strength could not be obtained, with cracks occurring when the activation treatment was carried out, making forming difficult. Further, the capacitance did not rise as much as anticipated.
Thus, active carbon electrodes formed using conventional methods as explained above have poor mechanical strength, making it impossible to obtain a large plate suitable for large current discharges.
Further, the capacitance of conventional active carbon polarizable electrodes when there is a strong discharge is less than half that when there is a weak discharge. Accordingly, the active carbon polarizable electrode of the conventional art presents a disadvantage in the case of strong discharges, and is not suitable for supplying a large current such as would be necessary for an auxiliary battery in a car.